This invention relates to a heating furnace in which articles are heated for drying, painting with baking, etc., and more particularly to a natural convection type heating furnace which can improve rising characteristics at an operation start-up time to stabilize the condition within the heating furnace in a shorter time.
In heating furnaces of this type, appropriate heating sources are selected in view of heating efficiency, stability, uniformity and safeness etc. and so on. A heating furnace, in which liquid or gas fuel is burnt using a burner unit and combustion gas is directly introduced to the heating furnace for heating the articles, is economically advantageous because of high heat calories and because the equipment is simple and easy to use.
However, because a large amount of combustion gas must be supplied at all times, it is required to exhaust the same quantity of gas. The degree of heat efficiency of the heating furnace is determined by how heat calories taken away with the exhaust is reduced. As one preferable heating furnace, there is known a natural convection type heating furnace wherein hot air, i.e., combustion gas, is successively introduced to a mountain-shaped furnace body like a vessel laid upside down, and the gas having a temperature reduced after heating the articles in the furnace body is forced to flow out from below the furnace body. Such a heating furnace is disclosed in Japanese Patent Publication No. 58-12513, No. 59-42225 and No. 63-16673. All these heating furnaces have been invented by the inventors of this application.
The disclosed natural convection type heating furnaces have high heat efficiency and are very economical as a result of utilizing the natural convection process that hot air at a high temperature resides in an upper portion of the furnace body and the gas having a temperature reduced after heating articles in the furnace body is forced to descend due to the difference in density and then discharge out from below the furnace body.
In the prior art natural convection type heating furnaces, however, since the burner unit is installed under the furnace body, the degree of freedom in design is relatively small and a belt conveyor is hard to install over the lower floor surface. It is also difficult to effectively utilize the radiant heat generated with the combustion in the burner unit.
Meanwhile, there is a hot air circulating furnace wherein combustion gas burnt in a burner unit is forcibly introduced to a heating furnace by a circulation pump through an inlet duct for constant circulation of the gas. However, this hot air circulating furnace provides problems in that it requires a long rising time until the interior of the heating furnace reaches a uniform heating temperature condition, and also lacks rising stability in the start-up operation.
An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art and to provide a heating furnace of natural convection type which can improve rising characteristics at the operation start-up time to stabilize the condition within the heating furnace in a shorter time and can maintain a uniform temperature in the furnace.
Another object of this invention is to provide a natural convection type heating furnace which can increase heating efficiency by effectively utilizing not only directly transferred heat but also radiant heat from a heating source and can maintain a stable and uniform heating space without causing significant fluctuations in heating surfaces within a furnace body.
Still another object of this invention is to provide a natural convection type heating furnace wherein a heating source is concentratedly arranged in a side or top portion of the furnace body to facilitate the maintenance thereof.
These and other objects can be achieved according to the present invention by providing a heating furnace of natural convection type comprising:
a furnace body having an inner heating chamber and provided with an inlet port through which an article to be heated is conveyed into the furnace body and an outlet port through which the article is discharged from the furnace body;
a conveyer means for supporting the article to be heated and conveying the article through the heating chamber of the furnace body from the inlet port to the outlet port;
a heating means disposed to the furnace body on at least one of side portion and top side portion of the furnace body at which the heating means is exposed to an interior of the heating chamber;
an exhaust means formed to a bottom portion of the furnace body with a predetermined distance from the heating means, said exhaust means having an upper end positioned below the article to be heated in the heating chamber of the furnace body; and
an air inlet port provided below the exhaust means in association with the heating means.
In preferred embodiments, the exhaust means comprises a plurality of exhaust ports disposed apart from each other in a longitudinal direction of the furnace body between the inlet port and the outlet port and said heating means comprises a plurality of heating sources disposed apart from each other between the adjacent exhaust ports.
The heating means comprises infrared lamps, Nichrome heaters or gaseous infrared radiators.
An an air filter is disposed at a location of the air inlet port. Hoods are disposed in vicinities of the inlet port and the outlet port of the furnace body and discharge means are connected to the hoods.
The furnace body has a substantially straight structure or mount-shape structure in a longitudinal direction thereof.
Shield plates are formed to the inlet port and the outlet port so that the shield plates extend downward from longitudinal end portions of the top side portion of the furnace body to close upper predetermined portions of the inlet and outlet ports.
An air curtain means is disposed in the heating chamber in vicinities of the inlet port and the outlet port.
According to the structures of the present invention described above, in the heating furnace of this invention, as set forth above, the heating source is provided in at least one of side and top portions of the furnace body such that the heating source is exposed to the interior of the heating chamber. Therefore, articles can be heated by utilizing not only directly transferred heat but also radiant heat from the heating source. It is thus possible to increase heating efficiency, improve rising characteristics at the start-up time, and stabilize the condition in the heating chamber in a shorter time.
Furthermore, even with the interior of the furnace body directly heated by the heating source, the heating source provides the moderate heating action by effectively utilizing a combination of directly transferred heat and radiant heat. As a result, the heating chamber kept at a stable and uniform temperature can be established without causing significant fluctuations in heating surfaces within the furnace body.
Still furthermore, the heating source is concentratedly arranged between the exhaust ports provided in spaced relation in the longitudinal direction of the furnace. Therefore, it is possible to conduct maintenance for the heating source in one lot and improve the operability of maintenance and inspection. Additionally, a belt conveyor may be installed on the lower floor surface of the furnace body, which results in the increased degree of freedom in design.
Still furthermore, the location or arrangement of the shielding plates and the air curtain means can achieve various merits as described hereinafter.